The disclosure relates generally to signal processing systems, and more particularly, to phase detection systems configured to measure phases of modulated signals.
The conventional approach for measuring a phase-modulated test signal with changing amplitude is to mix the test signal with a reference signal that is “very stable”. A very stable reference signal is one that has characteristics (frequency and amplitude) that are significantly more stable than the test signal itself. The voltage of the resulting output signal is proportional to the sine of the phase difference between the test signal and reference signal. However, this proportionality of the output signal, referred to as the calibration constant, still needs to be determined.
When the value of the phase changes between the reference signal and a respective input test signal, the deviation from a zero reference value is proportional to the difference in phase between the reference signal and the respective input signal. Accordingly, this proportionality can be maintained when the phase angle differential between the reference signal and the input test signal is relatively low, e.g., where difference is about 45 degrees. When, however, the difference approaches or exceeds 90 degrees, the amplitude of the mixed output signals are no longer proportional to the difference in phase between the reference signal and the respective input test signals.
Conventional phase measurement techniques, as described above, suffer from two major problems: they lose sensitivity when the total modulation angle is a multiple of 90 degrees and they have limited ability to distinguish between phase and amplitude modulation. Furthermore, the calibration constant can vary based on temperature and other environmental variables.